title: Regulation of extrinsic pathway factor Xa formation by tissue factor pathway inhibitor.
abstract: Tissue factor (TF) pathway inhibitor (TFPI) regulates factor X activation through the sequential inhibition of factor Xa and the VIIa.TF complex. Factor Xa formation was studied in a purified, reconstituted system, at plasma concentrations of factor X and TFPI, saturating concentrations of factor VIIa, and increasing concentrations of TF reconstituted into phosphatidylcholine:phosphatidylserine membranes (TF/PCPS) or PC membranes (TF/PC). The initial rate of factor Xa formation was equivalent in the presence or absence of 2.4 nM TFPI. However, reaction extent was small (<20%) relative to that observed in the absence of TFPI, implying the rapid inhibition of VIIa.TF during factor X activation. Initiation of factor Xa formation using increasing concentrations of TF/PCPS or TF/PC in the presence of TFPI yielded families of progress curves where both initial rate and reaction extent were linearly proportional to the concentration of VIIa.TF. These observations were consistent with a kinetic model in which the rate-limiting step represents the initial inhibition of newly formed factor Xa. Numerical analyses of progress curves yielded a rate constant for inhibition of VIIa.TF by Xa.TFPI (>10(8) M-1.s-1) that was substantially greater than the value (7.34 +/- 0.8 x 10(6) M-1.s-1) directly measured. Thus, VIIa.TF is inhibited at near diffusion-limited rates by Xa.TFPI formed during catalysis which cannot be explained by studies of the isolated reaction. We propose that the predominant inhibitory pathway during factor X activation may involve the initial inhibition of factor Xa either bound to or in the near vicinity of VIIa.TF on the membrane surface. As a result, VIIa.TF inhibition is unexpectedly rapid, and the concentration of active factor Xa that escapes regulation is linearly dependent on the availability of TF.
authors.affiliation: Department of Medicine, Division of Hematology/Oncology, Emory University, Atlanta, Georgia 30322, USA.
authors.forename: R J
authors.initials: RJ
authors.lastname: Baugh
authors.forename: G J
authors.initials: GJ
authors.lastname: Broze
authors.forename: S
authors.initials: S
authors.lastname: Krishnaswamy
mesh_terms: D000818:Animals; D002417:Cattle; D015951:Factor Xa / Q000096:biosynthesis*; D065427:Factor Xa Inhibitors; D006801:Humans; D007700:Kinetics; D008074:Lipoproteins / Q000502:physiology*; D008563:Membrane Lipids / Q000378:metabolism; D010713:Phosphatidylcholines / Q000378:metabolism; D010718:Phosphatidylserines / Q000378:metabolism
chemical_list: D065427:Factor Xa Inhibitors; D008074:Lipoproteins; D008563:Membrane Lipids; D010713:Phosphatidylcholines; D010718:Phosphatidylserines; C051928:lipoprotein-associated coagulation inhibitor; D015951:Factor Xa